IP Telephony Servers and PBXs

Many data-networking transactions are based on the concept of client-server computing.
Client computers make requests for services to server computers, which perform those services
and return the results. You’re probably familiar with Web servers, e-mail servers, and
database servers.
Adding voice data to IP networks provides yet another set of servers that are designed to
provide voice services in new and innovative ways. An IP PBX typically serves as the core
IP telephony server. On the PSTN, the PBX is often a “closed-box” system—it provides all
the voice functions and features you need, but usually in a proprietary manner. Management
of the closed-box platform is left up to the PBX vendor. With VoIP, an IP PBX can be
built on a PC platform running on an operating system like Microsoft Windows, Linux, or
Sun Solaris. While parts of the IP PBX are inherently proprietary, the platforms can be
managed through vendor application programming interfaces (APIs) and through the standard
APIs provided by the operating system itself.
An IP PBX provides functions and features like those that a traditional PBX provides.
While the standard PBX of the PSTN offers multiple features developed over decades, such
as call transfer and call forwarding, IP PBXs are quickly providing the same kinds of features
and more.

Other types of IP telephony servers provide new and interesting services. The possibility of
unified messaging—the convergence of voice mail and e-mail—can be considered a benefit
of a VoIP implementation. Unified messaging servers also run on PC platforms and talk to
e-mail servers and IP PBXs to provide message access in a variety of ways.